The present invention generally relates to semiconductor devices and more particularly to a pad electrode structure for use in semiconductor devices.
In semiconductor devices, it is necessary to form protrusion electrodes on pad electrodes for external connection, such that the pad electrodes, formed inside a semiconductor device on a semiconductor substrate, are connected to a substrate (such as interposer) used for mounting the semiconductor device thereon, electrically and also mechanically.
Generally, at the time of manufacture of semiconductor devices, it is necessary to ensure that each of the semiconductor devices shows electrically normal operation, upon completion of the manufacturing steps of the semiconductor device. For this purpose, it is practiced to carry out electric operational test by contacting a probe needle upon the pad electrodes formed on the semiconductor substrate.
In such operational test, it is necessary to press the probe needle against the pad electrode of Al or Cu, and thus, there is formed a flaw (hereinafter probe mark) on the surface of the pad electrode by the sharply pointed probe needle in such a manner that the surface of the pad electrode undergoes an irregular deformation.
FIG. 1 is a diagram of the semiconductor device for the case a bump is formed on the pad electrode carrying such a probe mark.
Referring to FIG. 1, a pad electrode 20 is formed on a semiconductor substrate 10 and a passivation film 30 is formed such that the pad electrode 20 is exposed. Further, a probe mark 40 is formed on the surface of the pad electrode 20 as a result of the operational test.
On such a pad electrode 20, a Ti layer 60 and a Cu layer 61 are formed by a sputtering method respectively as an adhesion layer and a conductive layer, with respective thicknesses of 300 nm and 250 nm. Further, a Ni layer 80 and an Au layer 90 are formed respectively with the thicknesses of 4000 nm and 200 nm, by conducting an electrolytic plating process while using the conductive layer 61 as an electrode. It should be noted that the Au layer 90 functions as an oxidation prevention film of the Ni layer 80.
Further, a bump electrode 100 is formed on the Au layer 90 by a lead-free solder such as the solder of the Sn—Ag system or a lead solder such as the solder of the An—Pb system.
In the example of FIG. 1, it should be noted that there is formed a probe mark 40 on the pad electrode 20 as a result of the operational test as explained before, and because of this, there can be a case in which no uniform formation of the adhesion layer 60 or the Cu layer 61 is made by the sputtering process, because of existence of the irregularity forming the probe mark 40. Because the Ti layer 60 or the Cu layer 61 is very thin, having the thickness of only 200–300 nm, it is not possible to achieve uniform film formation in the case there exists projections and depressions in the underlying layer as in this case.
Thus, in the case a Ni layer 80 and an Au layer 90 are formed by an electrolytic plating process while using the Cu layer 61 as an electrode, there occurs no growth of these layers on the probe mark 40, and thus, there can be a case, in the event a bump 100 is formed on the foregoing Au layer 90, that a void 110 is formed between the pad electrode 20 and the bump 100 in correspondence to the probe mark 40.
When there exists such a void 110 underneath the bump electrode 100, there occurs degradation of electrical or mechanical properties in the junction that uses such a bump electrode, and the reliability of the semiconductor device is degraded. Further, there can occur a problem that the metal element such as An, Ag, Pb, Ni, and the like, forming the material of the bump electrode, cause diffusion into the pad electrode 20, or Al forming the pad electrode 20 cause diffusion into the bump electrode 100, via the region where no such a Ti layer or Cu layer is formed. Thereby, increase of contact resistance is invited.
Thus, in the prior art, it was not possible to achieve a contact of the probe electrode directly on the electrode pad surface, and it has been practiced to carry out the electric operational test by providing a separate electrode pad on the semiconductor device for probe test. In such an approach, however, there occurs an increase in the area of the semiconductor device in view of the need of providing such separate pad electrode for the probe test, in addition to the pad electrodes on which the bump electrodes are formed.